Playing the right card – trade-offs between pre- and post-copulatory sexually selected traits in males of a highly sexually dimorphic mammal (#204)
We tested whether there is a trade-off between investment in pre- and post-copulatory competitive traits in male western grey kangaroos (Macropus fuliginosus). These kangaroos show indeterminate growth, with males reaching up to six times the body mass of adult females, and invest in greater forelimb musculature, which would aid in male-male contest, display to attract females, or female coercion1,2. In addition to body size and muscularity, male kangaroos also invest in post-copulatory competitive traits, including a 2.5 times increase in prostate mass during the breeding season, and positive allometry in accessory glands (bulbourethral glands) and the muscles that control erection of the penis. Consistent with an hypothesised trade-off between investment in pre- and post-copulatory competitive traits, less muscular males had increased sperm velocity; however muscularity was also positively correlated with investment in bulbourethral glands 2 and 3, and the penis and its muscles of erection, which does not support the concept of a trade-off in investment between pre- and post-copulatory competitive traits. Understanding the role of the accessory glands in sperm competition, and quantifying the relationship between behaviour and paternity may serve to elucidate the complex selection pressures that appear to be acting in this species.
- Jarman, P. J. (1989) Sexual dimorphism in Macropodoidea. In: Kangaroos, wallabies and rat-kangaroos, Vol. 1 (Grigg, G., Jarman, P. & Hume, I., eds.). pp. 433-447. Surrey Beatty & Sons Pty Ltd, Chipping Norton, NSW.
- Warburton, N. M., Bateman, PW, & Fleming, P. A. (2013). Sexual selection on forelimb muscles of western grey kangaroos (Skippy was clearly a female). Biological Journal of the Linnean Society, 109(4), 923-931.